1. Field of the Invention
The present invention relates to an observation system with an endoscope having an image guide fiber of a very small diameter.
2. Description of the Related Art
An ordinary observation system using an endoscope comprises an image guide fiber for transmitting an image of a subject, an object lens for imaging a subject located at an entrance end face side of an image guide, and an eyepiece lens system for observing an image, as an amplified image, at an exit end face of the image guide or a relay lens system for directing the image at a light reception surface of, for example, a CCD, etc. The object lens has an F number of 2 to 4 and a numerical aperture (NA) of about 0.25 to 0.125 at the exit side, these being determined by a trade-off between the depth of field and the brightness of a lighting system. The eyepiece lens or the relay lens has an F number of 2 and a numerical number (NA) of about 0.25 at the entrance side.
Recently, practical use has been made of an endoscope for a blood vessel which includes an insertion section of a small diameter of the order of a few hundred of .mu.m. In this type of endoscope, the brightness tends to be short due to the very small diameters of both an image guide and a light guide. Generally, for an optical system with a plurality of observation optical systems connected thereto, the brightness of the optical system is determined by the brightness of the lowest-illumination optical system.
In the optical system of this type, the numerical aperture (NA) at the exit side of the optical system located at the front side of an image guide for transmitting the subject image is made to correspond to the numerical aperture (NA) at the entrance side of the optical system located at the back side of the image guide. In a practical example, if the numerical aperture (NA) is 0.25 at the exit side of the object lens nearest the subject, the image guide is so designed as to be set to be over 0.25 and the numerical aperture is to be set to be 0.25 at the entrance side of the eyepiece lens or the relay lens. In this case, it has been common practice to impart a tolerance to the brightness of the back-side optical system, such as an image guide, eyepiece or relay lens, taking into consideration the assembly error, etc., of the optical system. In order to give more light to the optical system, it is necessary that the brightness levels of the respective optical systems be all increased.
For the small-diameter image guide it follows that a high amplification lens is necessarily employed as the eyepiece and relay lens. For this reason, the greater the numerical aperture at the entrance side of the eyepiece or the relay lens, much stricter the accuracy of component parts or accuracy of their assembly required.
Further, the image guide fiber comprises a common cladding and a great number of cores. Such an image fiber is manufactured in the following way. A greater number of relatively thick optical fibers called fiber elements are bundled and a resultant bundle, being heated, is spun into a fiber with a melted cladding set there.
It is desired that the image fiber be of a small diameter type. If, however, the core-to-core pitch (the center-to-center distance between the adjacent cores) is reduced below 10 .mu.m, a cross-talk occurs due to the wavy nature of light, thus causing a marked degradation of a propagating image in the fiber.
A so-called "random image fiber" including various kinds of different-diameter cores has been known as an image fiber aimed at reducing a cross-talk, that is, a degradation in an image. However, this type of fiber becomes expensive due to the use of a plurality of kinds of optical fibers upon manufacture.